Unify NixOS and Nixpkgs channel structure

This is primarily to ensure that

  -I nixpkgs=https://nixos.org/channels/nixos-unstable/nixexprs.tar.xz

and

  -I nixpkgs=https://github.com/NixOS/nixpkgs-channels/archive/nixos-unstable.tar.gz

and

  -I nixpkgs=https://nixos.org/channels/nixpkgs-unstable/nixexprs.tar.xz

behave consistently.

It also allows installing packages via "nix-env -iA nixos.<pkg>"
rather than "nixos.pkgs.<pkg>". It would be even better to allow
"nixpkgs.<pkg>", but that requires a change to nix-channel.

Fixes #7659.

nixos/modules/misc/extra-arguments.nix

@@ -2,7 +2,7 @@
 
 {
   _module.args = {
-    pkgs_i686 = import ../../lib/nixpkgs.nix {
+    pkgs_i686 = import ../../.. {
       system = "i686-linux";
       config.allowUnfree = true;
     };